Connected devices are increasingly reliant on internal time values to successfully carry out or regulate a variety of activities. The connected device may permit a person to carry out a particular device-related activity during a temporal window that the device monitors using an internal time value. As long as the current time value falls within the temporal window, the connected device will allow the activity to occur. However, when the internal time value indicates that the temporal window has expired, the connected device can stop allowing the activity.
In a temporally-based access system, a temptation exists to manipulate the internal time value of the connected device thereby allowing access outside of a defined temporal window. Sometimes, connected devices can be tricked into changing their internal time values in a way that a temporal window is always valid or never valid or such that the temporal window can be made valid in response to an unauthorized change to the internal time value of the connected device. While accurate time values can be obtained from signals broadcast by one or more GPS satellites, these time values may be susceptible to manipulation. The signals broadcast by the GPS satellites are not digitally signed or authenticated and a connected device relying on these signals to obtain a valid time can be fooled into receiving a maliciously-generated signal intended to mimic a GPS signal that includes a different time. It would be helpful to maintain an accurate time value at the connected device, but updates or modifications to the time value should be resistant to tampering or manipulation.